Among machine structural members, many of cylindrical members have conventionally been obtained from a steel bar into a desired shape by forging or elongating and rolling, or further by cutting, and thereafter heating bar to provide mechanical properties necessary for the machine structural member. In recent years, as structures tend to increase in size and in yield stress, an attempt has been made to reduce the weight of structure by replacing the cylindrical structural member with a hollow-shell seamless steel pipe. In particular, the steel pipe used as a cylindrical structural member such as a crane boom has been required to have high strength and high toughness in view of the increase in size of a crane, the operation on high-rise buildings and in cold districts, and the like. Recently, in the application to a boom, the seamless steel pipe has been required to have a tensile strength of 950 MPa or more and an excellent toughness at a temperature as low as −40° C. In such an application, the steel pipe having a wall thickness of about 5 to 50 mm, especially 8 to 45 mm, has been required in many cases.
As for the high-strength and high-toughness steel pipe, various techniques have conventionally been proposed.
For example, Patent Document 1 proposes a method for manufacturing a high-tension seamless steel pipe excellent in low-temperature toughness, in which a low-alloy steel containing C, Si, Mn, P, S, Ni, Cr, Mo, Ti, Al and N, and either or both of Nb and V, at predetermined content ranges, and further containing 0.0005 to 0.0025% of B is subjected to pipe-making and thereafter heat treated.
Patent Document 2 proposes a high-strength and high-toughness seamless steel pipe manufactured from a steel containing C, Si, Mn, P, S, Al, Nb and N, or further containing at least one selected from Cr, Mo, Ni, V, REM, Ca, Co and Cu, at predetermined content ranges, and further containing 0.0005 to 0.0030% of B, and furthermore containing Ti within the range of −0.005%<(Ti−3.4N)<0.01%, in which the size of the precipitate formed by precipitation due to tempering is 0.5 μm or less.
Also, Patent Document 3 proposes a technique for obtaining a high-strength seamless steel pipe by using a low-alloy steel containing C, Si, Mn, P, S, Al, Cr, Mo, V, Cu, N and W at predetermined content ranges to make a pipe, and by quenching and tempering the pipe.
Further, Patent Document 4 proposes a high-strength seamless steel pipe for machine structural use excellent in toughness and weldability, which is obtained by using a steel containing C, Mn, Ti and Nb at predetermined content ranges, and containing Si, Al, P, S and N so that the content ranges thereof are limited to predetermined limits or less, and further containing at least one selected from Ni, Cr, Cu and Mo, and furthermore containing 0.0003 to 0.003% of B, and by making a pipe by using the steel and thereafter subjecting the pipe to accelerated cooling and air cooling, so that the steel has a single self-tempered martensitic micro-structure or a mixed micro-structure of self-tempered martensitic micro-structure and lower bainite.